Refrigeration system with oil control system

ABSTRACT

A CO 2  refrigeration system includes a plurality of compressors configured to circulate a CO 2  refrigerant, a suction line configured to deliver the CO 2  refrigerant to the compressors, an oil separator configured to separate oil from the CO 2  refrigerant, and an oil return line configured to deliver the oil from the oil separator to the suction line. The oil mixes with the CO 2  refrigerant in the suction line before reaching the compressors.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of and priority to U.S. ProvisionalPatent Application No. 62/675,868 filed May 24, 2018, the entiredisclosure of which is incorporated by reference herein.

BACKGROUND

The present disclosure relates generally to a refrigeration system andmore particularly to a refrigeration system that uses carbon dioxide(i.e., CO₂) as a refrigerant. The present disclosure relates moreparticularly still to a CO₂ refrigeration system with an oil controlsystem.

Refrigeration systems are often used to provide cooling to temperaturecontrolled display devices (e.g. cases, merchandisers, etc.) insupermarkets and other similar facilities. Vapor compressionrefrigeration systems are a type of refrigeration system which providessuch cooling by circulating a fluid refrigerant (e.g., a liquid and/orvapor) through a thermodynamic vapor compression cycle. In a vaporcompression cycle, the refrigerant is typically compressed to a hightemperature high pressure state (e.g., by a compressor of therefrigeration system), cooled/condensed to a lower temperature state(e.g., in a gas cooler or condenser which absorbs heat from therefrigerant), expanded to a lower pressure (e.g., through an expansionvalve), and evaporated to provide cooling by absorbing heat into therefrigerant. CO₂ refrigeration systems are a type of vapor compressionrefrigeration system that use CO₂ as a refrigerant.

Some CO₂ refrigeration systems include an oil management system whichprovides oil to one or more compressors of the refrigeration system. Oilmanagement systems are typically either active or passive. Active oilcontrol systems use an oil separator to remove oil from the refrigerantand then provide the oil directly to each compressor (e.g., via an oilline connecting the oil separator to each compressor). Passive oilcontrol systems do not include an oil separator and allow the oil toremain mixed with the refrigerant throughout the refrigeration cycle.

This section is intended to provide a background or context to theinvention recited in the claims. The description herein may includeconcepts that could be pursued, but are not necessarily ones that havebeen previously conceived or pursued. Therefore, unless otherwiseindicated herein, what is described in this section is not prior art andis not admitted to be prior art by inclusion in this section.

SUMMARY

One implementation of the present disclosure is a CO₂ refrigerationsystem that provides cooling for a refrigeration load using carbondioxide (CO₂) as a refrigerant. The CO₂ refrigeration system includes aplurality of compressors configured to circulate the CO₂ refrigerant, asuction line configured to deliver the CO₂ refrigerant to thecompressors, an oil separator configured to separate oil from the CO₂refrigerant, and an oil return line configured to deliver the oil fromthe oil separator to the suction line. The oil mixes with the CO₂refrigerant in the suction line before reaching the compressors.

In some embodiments, the CO₂ refrigeration system includes a pluralityof oil equalization lines and a plurality of oil equalization valves.Each of the oil equalization lines may connect one of the compressors tothe oil return line. Each of the oil equalization valves may be locatedalong one of the oil equalization lines and configured to control a flowof oil through the oil equalization lines.

In some embodiments, the CO₂ refrigeration system includes a controllerconfigured to periodically open and close the plurality of oilequalization valves. Opening the oil equalization valves may cause anyexcess oil within the compressors to flow into the oil equalizationlines and equalizes an amount of oil within each of the compressors.

In some embodiments, the CO₂ refrigeration system includes an oilcontrol valve located along the oil return line and configured tocontrol a flow of oil from the oil separator to the suction line.

In some embodiments, the CO₂ refrigeration system includes an oil sensorconfigured to measure an amount of oil within the suction line.

In some embodiments, the CO₂ refrigeration system includes a controllerconfigured to open and close the oil control valve based on the measuredamount of oil within the suction line. The oil may be permitted to flowfrom the oil separator to the suction line when the oil control valve isopen and may be prevented from flowing from the oil separator to thesuction line when the oil control valve is closed.

In some embodiments, the controller is configured to open the oilcontrol valve in response to a determination that the measured amount ofoil within the suction line is less than an oil threshold and keep theoil control valve closed in response to a determination that themeasured amount of the oil within the suction line is greater than orequal to the oil threshold.

Another implementation of the present disclosure is a method foroperating a CO₂ refrigeration system that provides cooling for arefrigeration load using carbon dioxide (CO₂) as a refrigerant. Themethod includes operating a plurality of compressors to circulate theCO₂ refrigerant within the CO₂ refrigeration system, delivering the CO₂refrigerant to the compressors via a suction line coupled to an inlet ofthe compressors, separating oil from the CO₂ refrigerant at an oilseparator coupled to an outlet of the compressors via a discharge line,and delivering the oil from the oil separator to the suction line via anoil return line, wherein the oil mixes with the CO₂ refrigerant in thesuction line before reaching the compressors.

In some embodiments, the method includes returning excess oil from theplurality of compressors to the oil return line via a plurality of oilequalization lines. Each oil equalization line may connect one of thecompressors to the oil return line. The method may include operating aplurality of oil equalization valves to control a flow of the excess oilthrough the oil equalization lines. Each oil equalization valve may belocated along one of the oil equalization lines.

In some embodiments, the method includes periodically opening andclosing the plurality of oil equalization valves. Opening the oilequalization valves may cause any excess oil within the compressors toflow into the oil equalization lines and may equalize an amount of oilwithin each of the compressors.

In some embodiments, the method includes operating an oil control valvelocated along the oil return line to control a flow of oil from the oilseparator to the suction line.

In some embodiments, the method includes measuring an amount of oilwithin the suction line via an oil sensor coupled to the suction line.

In some embodiments, operating the oil control valve includes openingand closing the oil control valve based on the measured amount of oilwithin the suction line. The oil may be permitted to flow from the oilseparator to the suction line when the oil control valve is open and maybe prevented from flowing from the oil separator to the suction linewhen the oil control valve is closed.

In some embodiments, operating the oil control valve includes openingthe oil control valve in response to a determination that the measuredamount of oil within the suction line is less than an oil threshold andkeeping the oil control valve closed in response to a determination thatthe measured amount of the oil within the suction line is greater thanor equal to the oil threshold.

Another implementation of the present disclosure is a controller for aCO₂ refrigeration system that provides cooling for a refrigeration loadusing carbon dioxide (CO₂) as a refrigerant. The controller includes oneor more processors and one or more non-transitory computer-readablemedia storing instructions. When executed by the one or more processors,cause the one or more processors to perform operations includingoperating a plurality of compressors to circulate the CO₂ refrigerantwithin the CO₂ refrigeration system and obtaining a measurement of anamount of oil within a suction line coupled to an inlet of thecompressors. The suction line contains a mixture of oil and the CO₂refrigerant. The operations include comparing the amount of oil withinthe suction line to an oil threshold and operating an oil control valveto control a flow of oil from an oil separator to the compressor suctionline. The oil from the oil separator mixes with the CO₂ refrigerant inthe compressor suction line before reaching the compressors.

In some embodiments, operating the oil control valve includes openingthe oil control valve in response to a determination that the amount ofoil within the suction line is less than the oil threshold and keepingthe oil control valve closed in response to a determination that theamount of the oil within the suction line is greater than or equal tothe oil threshold.

In some embodiments, the operations include obtaining a secondmeasurement of the amount of oil within the suction line after openingthe oil control valve to release oil from the oil separator into thesuction line and deactivating one or more components of the CO₂refrigeration system in response to the second measurement of the amountof oil within the suction line being less than the oil threshold.

In some embodiments, the operations include opening a plurality of oilequalization valves connecting the plurality of compressors to an oilreturn line to equalize oil within each of the compressors and closingthe plurality of oil equalization valves after a predetermined amount oftime has elapsed.

In some embodiments, opening the plurality of oil equalization valvescauses any excess oil from the plurality of compressors to return to theoil return line via a plurality of oil equalization lines. Each oilequalization line may connect one of the compressors to the oil returnline.

In some embodiments, the oil return line receives both the flow of oilfrom the oil separator and a flow of oil from the oil equalization linesand delivers oil to the suction line.

The foregoing is a summary and thus by necessity containssimplifications, generalizations, and omissions of detail. Consequently,those skilled in the art will appreciate that the summary isillustrative only and is not intended to be in any way limiting. Otheraspects, inventive features, and advantages of the devices and/orprocesses described herein, as defined solely by the claims, will becomeapparent in the detailed description set forth herein and taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a CO₂ refrigeration system that includes ansemi-passive oil control system, according to an exemplary embodiment.

FIG. 2 is a block diagram of a controller configured to control the CO₂refrigeration system and oil control system of FIG. 1, according to anexemplary embodiment.

FIG. 3 is a flowchart of an oil control process which can be performedby the oil control system of FIG. 1 and the controller of FIG. 2,according to an exemplary embodiment.

DETAILED DESCRIPTION Overview

Referring generally to the FIGURES, a CO₂ refrigeration system is shown,according to various exemplary embodiments. The CO₂ refrigeration systemmay be a vapor compression refrigeration system which uses primarilycarbon dioxide (i.e., CO₂) as a refrigerant. In some implementations,the CO₂ refrigeration system is used to provide cooling for temperaturecontrolled display devices in a supermarket or other similar facility.

The CO₂ refrigeration system includes an oil control system. In someembodiments, the CO₂ refrigeration system and/or the oil control systeminclude some or all of the features described in U.S. Provisional PatentApplication No. 62/460,984 filed Feb. 20, 2017, the entire disclosure ofwhich is incorporated by reference herein. The oil control system mayinclude an oil separator configured to remove oil from the CO₂refrigerant. In some embodiments, the oil control system uses asemi-active or semi-passive oil control technique. Unlike active oilcontrol systems that provide oil directly to individual compressors, theoil control system may provide the oil from the oil separate into arefrigerant suction line that feeds into multiple compressors of the CO₂refrigeration system. The oil may mix with the CO₂ refrigerant in thesuction line before reaching the compressors.

In some embodiments, the oil control system includes several oilequalization lines, each of which connects a compressor of the CO₂refrigeration system to an oil return line. The oil equalization linesmay connect to the compressors at connection points defining a targetlevel of oil within each compressor. The oil control system may includeoil equalization valves located along each oil equalization line. Theoil equalization valves can be opened simultaneously to equalize theamount of oil within each compressor. When the oil equalization valvesare open, any oil within the compressors above the connection points mayflow into the oil equalization lines and into the oil return line,whereas any oil below the connection points may remain within thecompressors. The oil return line may then deliver any returned oil intothe compressor suction line to mix with the refrigerant.

The oil control system may include an oil sensor configured to measurean amount or level of oil within the compressor suction line. If themeasured amount of oil in the suction line is less than an oil thresholdafter the oil equalization procedure has been performed (e.g., twominutes later), an oil control valve may be opened to release oil fromthe oil separator into the oil return line, which delivers the oil tothe compressor suction line. However, if the measured amount of oil inthe suction line is greater than or equal to the oil threshold, the oilcontrol valve may remain closed. If the amount of oil in the suctionline does not reach the oil threshold after opening the oil controlvalve, an alarm may be generated and the CO₂ refrigeration system may beshutdown. These and other features of the CO₂ refrigeration system andthe oil control system are described in greater detail below.

CO₂ Refrigeration System

Referring now to FIG. 1, a CO₂ refrigeration system 100 is shown,according to an exemplary embodiment. CO₂ refrigeration system 100 maybe a vapor compression refrigeration system which uses primarily carbondioxide (CO₂) as a refrigerant. However, it is contemplated that otherrefrigerants can be substituted for CO₂ without departing from theteachings of the present disclosure. CO₂ refrigeration system 100 and isshown to include a system of pipes, conduits, or other fluid channels(e.g., fluid conduits 1, 3, 5, 7, 9, 13, 17, and 19) for transportingthe CO₂ refrigerant between various components of CO₂ refrigerationsystem 100. The components of CO₂ refrigeration system 100 are shown toinclude a gas cooler/condenser 2, a high pressure valve 4, a receiver 6,expansion valves 11, evaporators 12, and compressors 14.

Gas cooler/condenser 2 may be a heat exchanger or other similar devicefor removing heat from the CO₂ refrigerant. Gas cooler/condenser 2 isshown receiving CO₂ vapor from fluid conduit 1. In some embodiments, theCO₂ vapor in fluid conduit 1 may have a pressure within a range fromapproximately 45 bar to approximately 100 bar (i.e., about 640 psig toabout 1420 psig), depending on ambient temperature and other operatingconditions. In some embodiments, gas cooler/condenser 2 may partially orfully condense CO₂ vapor into liquid CO₂ (e.g., if system operation isin a subcritical region). The condensation process may result in fullysaturated CO₂ liquid or a liquid-vapor mixture (e.g., having athermodynamic quality between zero and one). In other embodiments, gascooler/condenser 2 may cool the CO₂ vapor (e.g., by removing superheat)without condensing the CO₂ vapor into CO₂ liquid (e.g., if systemoperation is in a supercritical region). In some embodiments, thecooling/condensation process is an isobaric process. Gascooler/condenser 2 is shown outputting the cooled and/or condensed CO₂refrigerant into fluid conduit 3.

High pressure valve 4 receives the cooled and/or condensed CO₂refrigerant from fluid conduit 3 and outputs the CO₂ refrigerant tofluid conduit 5. High pressure valve 4 may control the pressure of theCO₂ refrigerant in gas cooler/condenser 2 by controlling an amount ofCO₂ refrigerant permitted to pass through high pressure valve 4. In someembodiments, high pressure valve 4 is a high pressure thermal expansionvalve (e.g., if the pressure in fluid conduit 3 is greater than thepressure in fluid conduit 5). In such embodiments, high pressure valve 4may allow the CO₂ refrigerant to expand to a lower pressure state. Theexpansion process may be an isenthalpic and/or adiabatic expansionprocess, resulting in a flash evaporation of the high pressure CO₂refrigerant to a lower pressure, lower temperature state. The expansionprocess may produce a liquid/vapor mixture (e.g., having a thermodynamicquality between zero and one). In some embodiments, the CO₂ refrigerantexpands to a pressure of approximately 38 bar (e.g., about 540 psig),which corresponds to a temperature of approximately 37° F. The CO₂refrigerant then flows from fluid conduit 5 into receiver 6.

Receiver 6 collects the CO₂ refrigerant from fluid conduit 5. In someembodiments, receiver 6 may be a flash tank or other fluid reservoir.Receiver 6 includes a CO₂ liquid portion 16 and a CO₂ vapor portion 15and may contain a partially saturated mixture of CO₂ liquid and CO₂vapor. In some embodiments, receiver 6 separates the CO₂ liquid from theCO₂ vapor. The CO₂ liquid may exit receiver 6 through fluid conduit 9.Fluid conduit 9 may be liquid headers leading to expansion valves 11 andevaporators 12. The CO₂ vapor may exit receiver 6 through fluid conduit7. Fluid conduit 7 is shown leading the CO₂ vapor to fluid conduit 13.

In various embodiments, any number of expansion valves 11, evaporators12, and compressors 14 may be present. Expansion valves 11 may beelectronic expansion valves or other similar expansion valves. Expansionvalves 11 are shown receiving liquid CO₂ refrigerant from fluid conduit9 and outputting the CO₂ refrigerant to evaporators 12. Expansion valves11 may cause the CO₂ refrigerant to undergo a rapid drop in pressure,thereby expanding the CO₂ refrigerant to a lower pressure, lowertemperature state. In some embodiments, expansion valves 11 may expandthe CO₂ refrigerant to a pressure of approximately 30 bar. The expansionprocess may be an isenthalpic and/or adiabatic expansion process.

Evaporators 12 are shown receiving the cooled and expanded CO₂refrigerant from expansion valves 11. In some embodiments, evaporators12 may be associated with display cases/devices (e.g., if CO₂refrigeration system 100 is implemented in a supermarket setting).Evaporators 12 may be configured to facilitate the transfer of heat fromthe display cases/devices into the CO₂ refrigerant. The added heat maycause the CO₂ refrigerant to evaporate partially or completely.According to one embodiment, the CO₂ refrigerant is fully evaporated inevaporators 12. In some embodiments, the evaporation process may be anisobaric process. Evaporators 12 are shown outputting the CO₂refrigerant into discharge line 13, leading to a heat exchanger 20.

Heat exchanger 20 may be positioned within receiver 6 and configured totransfer heat between the CO₂ refrigerant entering heat exchanger 20from discharge line 13 and the CO₂ refrigerant entering receiver 6 fromfluid conduit 5. In some embodiments, heat exchanger 20 precools the CO₂refrigerant within receiver 6 by transferring heat from the CO₂refrigerant surrounding heat exchanger 20 into the CO₂ refrigerantwithin heat exchanger 20. The CO₂ refrigerant exiting heat exchanger 20may pass into suction line 17. In other embodiments, heat exchanger 20can be omitted and discharge line 13 may connect directly to suctionline 17. Suction line 17 may provide the CO₂ refrigerant to compressors14.

Compressors 14 may compress the CO₂ refrigerant into a superheated vaporhaving a pressure within a range of approximately 45 bar toapproximately 100 bar. The output pressure from compressors 14 may varydepending on ambient temperature and other operating conditions. In someembodiments, compressors 14 operate in a transcritical mode. Inoperation, the CO₂ discharge gas exits compressors 14 and flows intodischarge line 19. Discharge line 19 is shown providing the CO₂refrigerant to an oil separator 31, which separates oil from the CO₂refrigerant. The CO₂ refrigerant may exit oil separator 31 and flowthrough fluid conduit 1 into gas cooler/condenser 2. The oil separatedfrom the CO₂ refrigerant may flow into an oil return line 33.

Still referring to FIG. 1, CO₂ refrigeration system 100 is shown toinclude a gas bypass valve 8. Gas bypass valve 8 may receive the CO₂vapor from receiver 6 (via fluid conduit 7) and output the CO₂refrigerant into discharge line 13. In some embodiments, the CO₂ vaporthat is bypassed through gas bypass valve 8 is mixed with the CO₂refrigerant gas exiting evaporators 12 (e.g., via discharge line 13).The combined CO₂ refrigerant gas may be provided to the suction side ofcompressors 14. In other embodiments, gas bypass valve 8 may output theCO₂refrigerant into suction line 17. Compressors 14 may compress the CO₂vapor passing through gas bypass valve 8 from a low pressure state(e.g., approximately 30 bar or lower) to a high pressure state (e.g.,45-100 bar).

Gas bypass valve 8 may be operated to regulate or control the pressurewithin receiver 6 (e.g., by adjusting an amount of CO₂ refrigerantpermitted to pass through gas bypass valve 8). For example, gas bypassvalve 8 may be adjusted (e.g., variably opened or closed) to adjust themass flow rate, volume flow rate, or other flow rates of the CO₂refrigerant through gas bypass valve 8. Gas bypass valve 8 may be openedand closed (e.g., manually, automatically, by a controller 50, etc.) asneeded to regulate the pressure within receiver 6.

In some embodiments, gas bypass valve 8 includes a sensor for measuringa flow rate (e.g., mass flow, volume flow, etc.) of the CO₂ refrigerantthrough gas bypass valve 8. In other embodiments, gas bypass valve 8includes an indicator (e.g., a gauge, a dial, etc.) from which theposition of gas bypass valve 8 may be determined. This position may beused to determine the flow rate of CO₂ refrigerant through gas bypassvalve 8, as such quantities may be proportional or otherwise related.

In some embodiments, gas bypass valve 8 may be a thermal expansion valve(e.g., if the pressure on the downstream side of gas bypass valve 8 islower than the pressure in fluid conduit 7). According to oneembodiment, the pressure within receiver 6 is regulated by gas bypassvalve 8 to a pressure of approximately 38 bar, which corresponds toabout 37° F. Advantageously, this pressure/temperature state mayfacilitate the use of copper tubing/piping for the downstream CO₂ linesof the system. Additionally, this pressure/temperature state may allowsuch copper tubing to operate in a substantially frost-free manner.

Oil Control System

Still referring to FIG. 1, CO₂ refrigeration system 100 is shown toinclude an oil control system 30. Oil control system 30 can beconfigured to monitor and control the oil delivered to compressors 14.In some embodiments, the oil control performed by oil control system 30is semi-active or semi-passive. Active oil control systems typically usean oil separator to remove oil from the refrigerant and then provide theoil directly to each compressor (e.g., via an oil line connecting theoil separator to each compressor). Passive oil control systems typicallydo not include an oil separator and allow the oil to remain mixed withthe refrigerant throughout the refrigeration cycle. Advantageously, thesemi-active or semi-passive oil control performed by oil control system30 may remove oil from the CO₂ refrigerant, but does not provide the oildirectly to each compressor 14. Rather, the oil is returned to suctionline 17 via oil return line 33 and mixed with the CO₂ refrigerant beforethe CO₂ refrigerant is provided to compressors 14.

Oil control system 30 is shown to include an oil separator 31. Oilseparator 31 may be configured to separate oil from the compressed CO₂refrigerant. In some embodiments, oil separator 31 is positioneddownstream of compressors 14 (as shown in FIG. 1) such that oilseparator 31 receives the compressed CO₂ refrigerant from discharge line19. Oil separator 31 may remove oil from the compressed CO₂ refrigerantand may deliver the compressed CO₂ refrigerant into fluid conduit 1. Insome embodiments, the oil separated from the CO₂ refrigerant iscollected in oil separator 31. For example, the oil can be stored in aninternal reservoir within oil separator 31. In other embodiments, theoil can be stored in an external reservoir separate from oil separator31.

The oil separated from the CO₂ refrigerant by oil separator 31 may exitthe internal or external oil reservoir via oil return line 33. Oilreturn line 33 connects oil separator 31 and/or the oil reservoir tosuction line 17 where the oil mixes with the CO₂ refrigerant. Therefrigerant/oil mixture then feeds into compressors 14. An oil controlvalve 39 (e.g., a solenoid valve) may be positioned along oil returnline 33 and configured to control the flow of oil through oil returnline 33. Oil control valve 39 can be operated by controller 50(described in greater detail below) to control the release of oil fromoil separator 31 into suction line 17.

Unlike active oil control systems that provide oil directly toindividual compressors, oil control system 30 provides the oil into arefrigerant suction line 17 that feeds into multiple compressors 14 inparallel. The oil may mix with the CO₂ refrigerant in suction line 17before reaching compressors 14. The flow of oil back to compressors 14may be balanced such that each compressor 14 receives sufficient oil.

Still referring to FIG. 1, oil control system 30 is shown to includeseveral oil equalization lines 36 and oil equalization valves 37. Eachoil equalization line 36 connects one of compressors 14 to oil returnline 33 and is configured to deliver oil and/or CO₂ refrigerant from thecorresponding compressor 14 into oil return line 33. In someembodiments, oil equalization lines 36 are connected to the crankcasings of compressors 14. Oil equalization lines 36 may connect tocompressors 14 at connection points corresponding to the desired oillevel (e.g., the maximum allowable oil level or maximum desirable oillevel) within each of compressors 14. Accordingly, any oil withincompressors 14 above the desired oil level (i.e., above the connectionpoint) can flow into oil equalization lines 36 when valves 37 areopened, whereas any oil within compressors 14 below the desired oillevel (i.e., below the connection point) may remain within compressors14 when valves 37 are opened.

Each oil equalization valve 37 may be positioned along one of oilequalization lines 36 and can be operated (e.g., by controller 50) tocontrol the flow of oil through the corresponding oil equalization line36. In some embodiments, oil equalization valves 37 are periodicallyopened and closed by controller 50 to equalize the amount of oil withineach of compressors 14. For example, when compressors 14 are running,all oil equalization valves 37 may be opened simultaneously for apredetermined amount of time (e.g., ten seconds) and subsequently closedafter the predetermined amount of time has elapsed. If the oil levelwithin any of compressors 14 is above the connection point, any oilabove the connection point may flow into oil equalization lines 36,through oil equalization valves 37, and into oil return line 33 whilevalves 37 are open. If the level of oil within any of compressors 14 isbelow the connection point, any such compressors 14 may return only CO₂refrigerant gas through oil equalization lines 36 when valves 37 areopened. In this way, the level of oil within each of compressors 14 isequalized. The process of opening and closing oil equalization valves 37to equalize the amount of oil within compressors 14 is referred toherein as an oil equitation procedure.

In some embodiments, oil control system 30 includes an oil sensor 35.Oil sensor 35 may be located along suction line 17 and configured tomeasure the amount of oil within the refrigerant/oil mixture in suctionline 17. Oil sensor 35 can be any of a variety of sensor types (e.g.,optical, capacitive, dielectrical, resonance frequency, floating bulb,etc.) configured to sense the amount of oil in the refrigerant/oilmixture in suction line 17. In some embodiments, oil sensor 35 uses afrequency sweep technique to sense oil, either as a mist or as a liquidoil level, within suction line 17. For example, oil sensor 35 may be aBaumer frequency sweep sensor configured to sense the amount of oil inthe oil/refrigerant mixture within suction line 17.

In some embodiments, oil sensor 35 is used to sense the amount of oilwithin suction line 17 shortly after the oil equalization procedure isperformed. For example, oil sensor 35 may sense the amount of oil withinsuction line 17 approximately two minutes (or any other amount of time)after the oil equalization procedure is performed. Controller 50 maycompare the oil measurement from oil sensor 35 with a threshold oilvalue. If the oil measurement is greater than or equal to the thresholdoil value (i.e., the amount of oil within suction line 17 is above thethreshold), controller 50 may cause oil control valve 39 to remainclosed. However, if the oil measurement is less than the threshold oilvalue (i.e., the amount of oil within suction line 17 is below athreshold), controller 50 may cause oil control valve 39 to open for apredetermined amount of time (e.g., five seconds) to allow oil from oilseparator 31 to flow into oil return line 33. In other embodiments, oilcontrol valve 39 can be held open until the oil measurement obtained byoil sensor 35 reaches the threshold oil value rather than closing oilcontrol valve after the predetermined amount of time has elapsed.

If the oil measurement from oil sensor 35 fails to reach the thresholdoil value after opening oil control valve 39, controller 50 may generatean alarm indicating that insufficient oil is present within oil controlsystem 30. Upon generating the alarm, controller 50 may automaticallystop the operation of CO₂ refrigeration system 100 (e.g., by stoppingcompressors 14) to prevent any damage that could be caused byinsufficient oil.

Oil Controller

Referring now to FIG. 2, a block diagram illustrating controller 50 ingreater detail is shown, according to an exemplary embodiment.Controller 50 may receive signals from one or more measurement devices(e.g., pressure sensors, temperature sensors, flow sensors, etc.)located within CO₂ refrigeration system 100. For example, controller 50is shown receiving an oil measurement from oil sensor 35. Controller 50may also receive a compressor state signal from one or more ofcompressors 14. The compressor state signals may indicate which ofcompressors 14 are running and which of compressors 14 are not running.Controller 50 may use the oil measurement and the compressor states todetermine appropriate control actions for control devices of CO₂refrigeration system 100 (e.g., compressors 14, valves 4, 8, 11, 37, and39, flow diverters, power supplies, etc.).

In some embodiments, controller 50 is configured to operate gas bypassvalve 8 to maintain the CO₂ pressure within receiver 6 at a desiredsetpoint or within a desired range. In some embodiments, controller 50operates gas bypass valve 8 based on the temperature of the CO₂refrigerant at the outlet of gas cooler/condenser 2. In otherembodiments, controller 50 operates gas bypass valve 8 based a flow rate(e.g., mass flow, volume flow, etc.) of CO₂ refrigerant through gasbypass valve 8. Controller 50 may use a valve position of gas bypassvalve 8 as a proxy for CO₂ refrigerant flow rate. In some embodiments,controller 50 operates high pressure valve 4 and expansion valves 11 toregulate the flow of refrigerant in system 100. In some embodiments,controller 50 operates valves 37 and 39 to regulate the flow of oil inoil control system 30.

Controller 50 may include feedback control functionality for adaptivelyoperating the various components of CO₂ refrigeration system 100. Forexample, controller 50 may receive a setpoint (e.g., a temperaturesetpoint, a pressure setpoint, a flow rate setpoint, a power usagesetpoint, etc.) and operate one or more components of system 100 toachieve the setpoint. The setpoint may be specified by a user (e.g., viaa user input device, a graphical user interface, a local interface, aremote interface, etc.) or automatically determined by controller 50based on a history of data measurements. In some embodiments, controller50 includes some or all of the features of the controller described inP.C.T. Patent Application No. PCT/US2016/044164 filed Jul. 27, 2016,and/or U.S. Provisional Patent Application No. 62/460,984 filed Feb. 20,2017, the entire disclosures of which are incorporated by referenceherein.

Controller 50 may be a proportional-integral (PI) controller, aproportional-integral-derivative (PID) controller, a pattern recognitionadaptive controller (PRAC), a model recognition adaptive controller(MRAC), a model predictive controller (MPC), or any other type ofcontroller employing any type of control functionality. In someembodiments, controller 50 is a local controller for CO₂ refrigerationsystem 100. In other embodiments, controller 50 is a supervisorycontroller for a plurality of controlled subsystems (e.g., arefrigeration system, an AC system, a lighting system, a securitysystem, etc.). For example, controller 50 may be a controller for acomprehensive building management system incorporating CO₂ refrigerationsystem 100. Controller 50 may be implemented locally, remotely, or aspart of a cloud-hosted suite of building management applications.

Still referring to FIG. 2, controller 50 is shown to include acommunications interface 52 and a processing circuit 60. Communicationsinterface 52 can be or include wired or wireless interfaces (e.g.,jacks, antennas, transmitters, receivers, transceivers, wire terminals,etc.) for conducting electronic data communications. For example,communications interface 52 may be used to conduct communications withgas bypass valve 8, oil sensor 35, compressors 14, valves 11, 37, and39, high pressure valve 4, various data acquisition devices within CO₂refrigeration system 100 (e.g., temperature sensors, pressure sensors,flow sensors, etc.) and/or other external devices or data sources. Datacommunications may be conducted via a direct connection (e.g., a wiredconnection, an ad-hoc wireless connection, etc.) or a network connection(e.g., an Internet connection, a LAN, WAN, or WLAN connection, etc.).For example, communications interface 52 can include an Ethernet cardand port for sending and receiving data via an Ethernet-basedcommunications link or network. In another example, communicationsinterface 52 can include a Wi-Fi transceiver or a cellular or mobilephone transceiver for communicating via a wireless communicationsnetwork.

Processing circuit 60 is shown to include a processor 62 and memory 70.Processor 62 can be implemented as a general purpose processor, anapplication specific integrated circuit (ASIC), one or more fieldprogrammable gate arrays (FPGAs), a group of processing components, amicrocontroller, or other suitable electronic processing components.Memory 70 (e.g., memory device, memory unit, storage device, etc.) maybe one or more devices (e.g., RAM, ROM, solid state memory, hard diskstorage, etc.) for storing data and/or computer code for completing orfacilitating the various processes, layers and modules described in thepresent application. Memory 70 may be or include volatile memory ornon-volatile memory. Memory 70 may include database components, objectcode components, script components, or any other type of informationstructure for supporting the various activities and informationstructures described in the present application. According to anexemplary embodiment, memory 70 is communicably connected to processor62 via processing circuit 60 and includes computer code for executing(e.g., by processing circuit 60 and/or processor 62) one or moreprocesses or control features described herein.

Still referring to FIG. 2, controller 50 is shown to include an oildetector 72 and a compressor state detector 74. Oil detector 72 can beconfigured to detect the level or amount of oil within suction line 17based on the oil measurement from oil sensor 35. For example, oildetector 72 can determine whether the amount or level of oil within theoil/refrigerant mixture in suction line 17 exceeds a threshold oillevel. Compressor state detector 74 can be configured to detect thestate of each of compressors 14. For example, compressor state detector74 can determine whether each of compressors 14 is currently running ornot running.

Controller 50 is shown to include a valve controller 76. Valvecontroller 76 may receive input from oil detector 72 indicating thecurrent oil amount or level within suction line 17. Valve controller 76may also receive input from compressor state detector 74 indicating thecurrent state of each of compressors 14. Valve controller 76 cangenerate control signals for one or more of valves 4, 8, 11, 37, and 39based on the current oil amount/level and/or the current state ofcompressors 14.

In some embodiments, valve controller 76 causes oil equalization valves37 to open and close periodically. For example, valve controller 76 cancause oil equalization valves 37 to open for a predetermined amount oftime (e.g., ten seconds) at a periodic interval (e.g., every thirtyminutes). The predetermined amount of time and the periodic interval areadjustable and can be set to any amount of time, as may be desirable invarious implementations. In some embodiments, valve controller 76 causesall of oil equalization valves 37 to open simultaneously. After thepredetermined amount of time has expired, valve controller 76 may causeoil equalization valves 37 to close simultaneously. As described above,opening and closing oil equalization valves 37 may equalize the amountof oil within each of compressors 14.

In some embodiments, valve controller 76 uses the oil measurement fromoil sensor 35 to generate control signals for oil control valve 39. Forexample, valve controller 76 can compare the oil measurement from oilsensor 35 to an oil threshold. Valve controller 76 may open oil controlvalve 39 in response to a determination that the amount of oil withinsuction line 17 is less than the oil threshold. Opening oil controlvalve 39 may cause oil to be dispensed from oil separator 31 and flowinto suction line 17 via oil return line 33. However, if the amount ofoil within suction line 17 is greater than or equal to the oilthreshold, valve controller 76 may cause oil control valve 39 to remainclosed.

Oil Control Process

Referring now to FIG. 3, a block diagram of an oil control process 80 isshown, according to an exemplary embodiment. Process 80 can be performedby one or more components of CO₂ refrigeration system 100. In someembodiments, process 80 is performed by controller 50, as described withreference to FIGS. 1-2.

Process 80 is shown to include opening oil equalization valves 37connecting compressors 14 to compressor suction line 17 to equalize oilwithin each compressor 14 (step 81) and closing oil equalization valves37 after a predetermined amount of time has elapsed (step 82). Step 81may include opening oil equalization valves 37 simultaneously to allowoil from within each compressor 14 to flow through oil equalizationlines 36 and into oil return line 33. When oil equalization valves 37are open, any excess oil within compressors 14 may flow into oilequalization lines 36 and into oil return line 33. Excess oil mayinclude any oil within compressors 14 above the point at which oilequalization lines 37 connect to compressors 14. Any oil withincompressors 14 below the connection point may remain in compressors 14.In some embodiments, steps 81 and 82 are performed periodically (e.g.,every ten minutes, every thirty minutes, etc.). The predetermined amountof time for which valves 37 are held open may be configurable and can beset to any desired value (e.g., ten seconds, five seconds, thirtyseconds, etc.)

Process 80 is shown to include measuring an amount of oil within acompressor suction line 17 containing a mixture of oil and refrigerant(step 83). In some embodiments, step 83 is performed by oil sensor 35.Oil sensor 35 can sense the amount of oil within suction line 17 and canprovide an oil measurement to controller 50. In some embodiments, step83 is performed a predetermined amount of time (e.g., two minutes) afteroil equalization valves 37 are opened and closed. For example, process80 may include waiting for two minutes (or any other amount of time)after step 82 is performed before advancing to step 83. This may allowthe oil within suction line 17 to equilibrate before oil sensor 35 isused to measure the amount of oil.

Process 80 is shown to include comparing the measured amount of oil toan oil threshold (step 84). If the measured amount of oil within suctionline 17 is not less than the oil threshold (i.e., the measured amount ofoil is greater than or equal to the oil threshold), controller 50 maycause oil control valve 39 to remain closed (step 85) and process 80 mayreturn to step 81 at the next equalization time (e.g., after thirtyminutes). However, if the measured amount of oil within suction line 17is less than the oil threshold, controller 50 may cause oil controlvalve 39 to open to release oil from oil separator 31 and into suctionline 17 via oil return line 33 (step 86). In some embodiments, oilcontrol valve 39 is held open for a predetermined amount of time (e.g.,five seconds, ten seconds, etc.) and then closed. In other embodiments,oil control valve 39 is held open until the measured amount of oilreaches the oil threshold.

After opening oil control valve 39, the amount of oil within suctionline 17 can be measured again and compared with the oil threshold (step87). In some embodiments, step 87 is performed a predetermined amount oftime (e.g., two minutes) after oil control valve 39 is opened. Forexample, process 80 may include waiting for two minutes (or any otheramount of time) after step 86 is performed before advancing to step 87.This may allow the oil within suction line 17 to equilibrate before oilsensor 35 is used to measure the amount of oil.

If the measured amount of oil within suction line 17 is not less thanthe oil threshold (i.e., the measured amount of oil is greater than orequal to the oil threshold), controller 50 may determine that there issufficient oil in system 100 and process 80 may return to step 81.However, if the measured amount of oil within suction line 17 is lessthan the oil threshold, controller 50 may generate an alarm indicatinginsufficient oil and may shutdown system 100 (step 88). Process 80 maybe repeated again at the next equalization time (e.g., thirty minuteslater or at any other periodic interval).

Configuration of Exemplary Embodiments

The construction and arrangement of the CO₂ refrigeration system asshown in the various exemplary embodiments are illustrative only.Although only a few embodiments have been described in detail in thisdisclosure, those skilled in the art who review this disclosure willreadily appreciate that many modifications are possible (e.g.,variations in sizes, dimensions, structures, shapes and proportions ofthe various elements, values of parameters, mounting arrangements, useof materials, colors, orientations, etc.) without materially departingfrom the novel teachings and advantages of the subject matter describedherein. For example, elements shown as integrally formed may beconstructed of multiple parts or elements, the position of elements maybe reversed or otherwise varied, and the nature or number of discreteelements or positions may be altered or varied. The order or sequence ofany process or method steps may be varied or re-sequenced according toalternative embodiments. Other substitutions, modifications, changes andomissions may also be made in the design, operating conditions andarrangement of the various exemplary embodiments without departing fromthe scope of the present invention.

As utilized herein, the terms “approximately,” “about,” “substantially”,and similar terms are intended to have a broad meaning in harmony withthe common and accepted usage by those of ordinary skill in the art towhich the subject matter of this disclosure pertains. It should beunderstood by those of skill in the art who review this disclosure thatthese terms are intended to allow a description of certain featuresdescribed and claimed without restricting the scope of these features tothe precise numerical ranges provided. Accordingly, these terms shouldbe interpreted as indicating that insubstantial or inconsequentialmodifications or alterations of the subject matter described and claimedare considered to be within the scope of the invention as recited in theappended claims.

It should be noted that the term “exemplary” as used herein to describevarious embodiments is intended to indicate that such embodiments arepossible examples, representations, and/or illustrations of possibleembodiments (and such term is not intended to connote that suchembodiments are necessarily extraordinary or superlative examples).

The terms “coupled,” “connected,” and the like as used herein mean thejoining of two members directly or indirectly to one another. Suchjoining may be stationary (e.g., permanent) or moveable (e.g., removableor releasable). Such joining may be achieved with the two members or thetwo members and any additional intermediate members being integrallyformed as a single unitary body with one another or with the two membersor the two members and any additional intermediate members beingattached to one another.

References herein to the positions of elements (e.g., “top,” “bottom,”“above,” “below,” etc.) are merely used to describe the orientation ofvarious elements in the FIGURES. It should be noted that the orientationof various elements may differ according to other exemplary embodiments,and that such variations are intended to be encompassed by the presentdisclosure.

The present disclosure contemplates methods, systems and programproducts on memory or other machine-readable media for accomplishingvarious operations. The embodiments of the present disclosure may beimplemented using existing computer processors, or by a special purposecomputer processor for an appropriate system, incorporated for this oranother purpose, or by a hardwired system. Embodiments within the scopeof the present disclosure include program products or memory includingmachine-readable media for carrying or having machine-executableinstructions or data structures stored thereon. Such machine-readablemedia can be any available media that can be accessed by a generalpurpose or special purpose computer or other machine with a processor.By way of example, such machine-readable media can comprise RAM, ROM,EPROM, EEPROM, CD-ROM or other optical disk storage, magnetic diskstorage or other magnetic storage devices, or any other medium which canbe used to carry or store desired program code in the form ofmachine-executable instructions or data structures and which can beaccessed by a general purpose or special purpose computer or othermachine with a processor. Combinations of the above are also includedwithin the scope of machine-readable media. Machine-executableinstructions include, for example, instructions and data which cause ageneral purpose computer, special purpose computer, or special purposeprocessing machines to perform a certain function or group of functions.

Although the figures may show a specific order of method steps, theorder of the steps may differ from what is depicted. Also two or moresteps may be performed concurrently or with partial concurrence. Suchvariation will depend on the software and hardware systems chosen and ondesigner choice. All such variations are within the scope of thedisclosure. Likewise, software implementations could be accomplishedwith standard programming techniques with rule based logic and otherlogic to accomplish the various connection steps, processing steps,comparison steps and decision steps.

What is claimed is:
 1. A CO₂ refrigeration system that provides coolingfor a refrigeration load using carbon dioxide (CO₂) as a refrigerant,the CO₂ refrigeration system comprising: a plurality of compressorsconfigured to circulate the CO₂ refrigerant within the CO₂ refrigerationsystem; a suction line configured to deliver the CO₂ refrigerant to thecompressors; an oil separator configured to separate oil from the CO₂refrigerant; and an oil return line configured to deliver the oil fromthe oil separator to the suction line, wherein the oil mixes with theCO₂ refrigerant in the suction line before reaching the compressors. 2.The CO₂ refrigeration system of claim 1, further comprising: a pluralityof oil equalization lines, each connecting one of the compressors to theoil return line; and a plurality of oil equalization valves, eachlocated along one of the oil equalization lines and configured tocontrol a flow of oil through the oil equalization lines.
 3. The CO₂refrigeration system of claim 2, further comprising a controllerconfigured to periodically open and close the plurality of oilequalization valves; wherein opening the oil equalization valves causesany excess oil within the compressors to flow into the oil equalizationlines and equalizes an amount of oil within each of the compressors. 4.The CO₂ refrigeration system of claim 1, further comprising an oilcontrol valve located along the oil return line and configured tocontrol a flow of oil from the oil separator to the suction line.
 5. TheCO₂ refrigeration system of claim 4, further comprising an oil sensorconfigured to measure an amount of oil within the suction line.
 6. TheCO₂ refrigeration system of claim 5, further comprising a controllerconfigured to open and close the oil control valve based on the measuredamount of oil within the suction line; wherein the oil is permitted toflow from the oil separator to the suction line when the oil controlvalve is open and prevented from flowing from the oil separator to thesuction line when the oil control valve is closed.
 7. The CO₂refrigeration system of claim 6, wherein the controller is configuredto: open the oil control valve in response to a determination that themeasured amount of oil within the suction line is less than an oilthreshold; and keep the oil control valve closed in response to adetermination that the measured amount of the oil within the suctionline is greater than or equal to the oil threshold.
 8. A method foroperating a CO₂ refrigeration system that provides cooling for arefrigeration load using carbon dioxide (CO₂) as a refrigerant, themethod comprising: operating a plurality of compressors to circulate theCO₂ refrigerant within the CO₂ refrigeration system; delivering the CO₂refrigerant to the compressors via a suction line coupled to an inlet ofthe compressors; separating oil from the CO₂ refrigerant at an oilseparator coupled to an outlet of the compressors via a discharge line;and delivering the oil from the oil separator to the suction line via anoil return line, wherein the oil mixes with the CO₂ refrigerant in thesuction line before reaching the compressors.
 9. The method of claim 8,further comprising: returning excess oil from the plurality ofcompressors to the oil return line via a plurality of oil equalizationlines, each oil equalization line connecting one of the compressors tothe oil return line; and operating a plurality of oil equalizationvalves to control a flow of the excess oil through the oil equalizationlines, each oil equalization valve located along one of the oilequalization lines.
 10. The method of claim 9, further comprisingperiodically opening and closing the plurality of oil equalizationvalves; wherein opening the oil equalization valves causes any excessoil within the compressors to flow into the oil equalization lines andequalizes an amount of oil within each of the compressors.
 11. Themethod of claim 8, further comprising operating an oil control valvelocated along the oil return line to control a flow of oil from the oilseparator to the suction line.
 12. The method of claim 11, furthercomprising measuring an amount of oil within the suction line via an oilsensor coupled to the suction line.
 13. The method of claim 12, whereinoperating the oil control valve comprises opening and closing the oilcontrol valve based on the measured amount of oil within the suctionline; wherein the oil is permitted to flow from the oil separator to thesuction line when the oil control valve is open and prevented fromflowing from the oil separator to the suction line when the oil controlvalve is closed.
 14. The method of claim 13, wherein operating the oilcontrol valve comprises: opening the oil control valve in response to adetermination that the measured amount of oil within the suction line isless than an oil threshold; and keeping the oil control valve closed inresponse to a determination that the measured amount of the oil withinthe suction line is greater than or equal to the oil threshold.
 15. Acontroller for a CO₂ refrigeration system that provides cooling for arefrigeration load using carbon dioxide (CO₂) as a refrigerant, thecontroller comprising: one or more processors; and one or morenon-transitory computer-readable media storing instructions that, whenexecuted by the one or more processors, cause the one or more processorsto perform operations comprising: operating a plurality of compressorsto circulate the CO₂ refrigerant within the CO₂ refrigeration system;obtaining a measurement of an amount of oil within a suction linecoupled to an inlet of the compressors, the suction line containing amixture of oil and the CO₂ refrigerant; comparing the amount of oilwithin the suction line to an oil threshold; and operating an oilcontrol valve to control a flow of oil from an oil separator to thecompressor suction line, the oil from the oil separator mixing with theCO₂ refrigerant in the compressor suction line before reaching thecompressors.
 16. The controller of claim 15, wherein operating the oilcontrol valve comprises: opening the oil control valve in response to adetermination that the amount of oil within the suction line is lessthan the oil threshold; and keeping the oil control valve closed inresponse to a determination that the amount of the oil within thesuction line is greater than or equal to the oil threshold.
 17. Thecontroller of claim 16, the operations further comprising: obtaining asecond measurement of the amount of oil within the suction line afteropening the oil control valve to release oil from the oil separator intothe suction line; and deactivating one or more components of the CO₂refrigeration system in response to the second measurement of the amountof oil within the suction line being less than the oil threshold. 18.The controller of claim 15, the operations further comprising: opening aplurality of oil equalization valves connecting the plurality ofcompressors to an oil return line to equalize oil within each of thecompressors; and closing the plurality of oil equalization valves aftera predetermined amount of time has elapsed.
 19. The controller of claim18, wherein opening the plurality of oil equalization valves causes anyexcess oil from the plurality of compressors to return to the oil returnline via a plurality of oil equalization lines, each oil equalizationline connecting one of the compressors to the oil return line.
 20. Thecontroller of claim 18, wherein the oil return line receives both theflow of oil from the oil separator and a flow of oil from the oilequalization lines and delivers oil to the suction line.